Help with Standel 25L15 Preamp Circuit

[10thTx]

As an FYI there is a reasonably significant amount of information on this amp on the Hoffman forum. IF you go there just do a "search". Several individuals have attempted clones and shared their results.

Some fairly long threads on this. In case you are wondering, I do NOT have any inside knowledge of this amp.

http://www.el34world.com/Forum/index.php?topic=14263.0

http://www.el34world.com/Forum/index.php?topic=15542.0

http://www.el34world.com/Forum/index.php?topic=16071.0

With respect, 10thtx

[Structo]

You built a beautiful cab there.

Someday I hope to be able to build something as nice.

[Cameron]

Thanks for responding guys!

Cameron - you drew that schematic that I attached? Am I hearing you right? What was your affiliation with Bob Crooks, the way you speak of him it seems that you worked with him. If so - maybe you could tell me what is going on electrically with the 12AT7 - that tone stack is very strange. I think "rsi" is right - there is positive feedback going on. That would explain the shape on the scope. I also started at the 12AX7 and marked where the phase gets inverted (from gain stages) - all the way up to to the cathode of the the first triode of 12AT7. It seems that the signals are the same phase (indicating positive feedback) and NOT inverted like a normal negative feedback would be. Is this the way he designed it? What was the reasoning? How does this work?

Unfortunately I don't have any 250 pf caps on hand but I did throw a 0.001 uF cap right after the 0.047uF cap right to ground. I also tied 2 caps in parallel to get 0.0005 uF (500pf) and I didn't notice much. It didn't seem to help the nasty sounding distortion under very heavy signals (such as strumming). What exactly is the purpose of this cap? A friend of mine who reverse engineered an original 25L15 did not find that cap after the 0.047 uF cap - it just goes right to the 270K. I am not doubting you, I just want to know more!

Structo - this is the preamp to a whole guitar amp - all the way up to the cathodyne phase inverter. The power amp and power supply sections are on a different schematic.

The .001 should have done it if that was the problem.....is it in the power amp or preamp....did you check both? ...check the cables to the power amp if you built it that way.

I did the reissues of the Standell 25l15 with Danny. I built the prototype and the first few of 25l15 reissue amps as well as a bunch of custom amps. Doug Aldrich used the prototype high gain amp I made....You can see it in his rack from the Dio tour. I recently bought the rest of the high gain amps that I made when I was there ...about 10.......Ill be finishing these up and and putting them up on ebay soon.
There is one of my schematics out there if you Google....it looks like its on a napkin. You can find most of the info online now. If you look at it simply .....its like feed back in a power amp. The bass control is kind of like a depth control and the treble is like a presence control that you would find in a Marshall power amp.

The cap I added when I found some funny stuff going on when I cranked it up...like you where saying....so I thought I would throw it out there. Its a clean amp its not meant to be distorted....Bob would not have liked that

[M Fowler]

The old Weber kits Gaylon 807 and JP25 were suppose to be based on 25L15.

The 807 is 60w version voiced for pedal steel and JP25 is 25w version.

[Cameron]

The old Weber kits Gaylon 807 and JP25 were suppose to be based on 25L15.

The 807 is 60w version voiced for pedal steel and JP25 is 25w version.

Nothing Standell about them ...I did not understand why these where said to be a Standell circuit.

[JazzGuitarGimp]

Wow, look at that. A pair of 807's in a guitar amp! What's the voltage at PS node A?

[M Fowler]

The old Weber kits Gaylon 807 and JP25 were suppose to be based on 25L15.

The 807 is 60w version voiced for pedal steel and JP25 is 25w version.

Nothing Standell about them ...I did not understand why these where said to be a Standell circuit.

I don't know just reporting what Weber listed on the PDF I included above.

807 60 watt varation on a Standell 25L15, tweaked specifically for Pedal
Steel

JP25 25 watt Standel 25L15 with Williamson Output

[10thTx]

From someone (not me) who has seen an original Standel schematic, it has been reported to me that the Weber is not close to being the same.

With respect, 10thtx

[M Fowler]

Not close but still information to share and since most never build exact clones people can add this to their collection of schematics.

I don't see that all of the 25L15 schematics out there are accurate either. They are using different voltages and transformers etc.

[Cameron]

From someone (not me) who has seen an original Standel schematic, it has been reported to me that the Weber is not close to being the same.

With respect, 10thtx

The schematics online are Standell's..this one is the one I made for my notes.
I don't know how they got out..Danny was trying to keep them a secret for as long as he could.

[Cameron]

One other piece of information that I thought I would throw in here is that I do have a reissue Stanel 25L15. It is one of the early ones with 6550 output tubes and also reverb and effects loop. Yes the circuit is quite different - but it still sounds really great. Now when I push the amp around 6 on the volume the distortion actually sounds really good! There is no static-y sounding distortion at all. So that leads me to believe there is something fishy going on in the amps that I built and with that hand drawn schematic that I attached.

I also added grid stopper resistors before each grid and that didn't help it - it may have smoothed it out a bit more, but it's still there.

I made this amp .....its not a 25l15 ...its a bit different....as you can see.. Does it have the tube loop or just the insert? Can you post a pic of it? So I can see which one it is.

[ubxf]

I have a couple of questions regarding that preamp , what is the function of the pot and jack that are on the opposite side of the input and all the control pots (bottom of the chassis). I tried to open it up( i removed the 6 screws on the side) but the chassis looked like it was glued, is there a trick.
Many thanks in advance for any info and suggestions.
Btw my preamp has only 2 tubes